A worldwide helium shortage has developed in recent years due to the limited supply of 3He and the increasing demand for neutron detection materials for scientific and security applications. As a result, research into alternatives to gas detectors (3He or 10BF3) and scintillation detectors (cerium-doped lithium glass) has expanded. It is highly desirable to develop a solid-state detector for neutron detection. In much the same way as solid-state CZT detectors have revolutionized gamma-ray detection, a solid-state neutron detector would offer the significant advantages of portability, sensitivity, simplicity, and low cost. A neutron absorber (e.g., 6Li or 10B) must be used along with a charge collector in such a device. To date, most lithium-containing solid-state neutron detectors have utilized a lithium conversion layer in conjunction with a silicon diode detector. To obtain a useable thickness of lithium to stop neutrons efficiently, deep holes are etched in the silicon, and a lithium (or boron)-containing material is conformally deposited in the holes. If, however, the neutron absorber is within the charge generating/collecting device, which is the semiconductor, each thermal neutron impinging on the detector crystal has a high probability of reacting with the absorber atom inside the solid, generating high-energy charged particles that, in turn, use their energies to create electron-hole pairs in the semiconductor. This is the same phenomenon that occurs in coated semiconductor detectors; however, in the Li-containing compound semiconductors, the charged particles are created within the charge collection device, and the entire Q-value of the reaction is available for charge generation. In the coated detectors, however, only one of the two charged particles can enter the detector, and the alpha particle must have lost a random fraction of its energy traversing the absorber layer. Ternary AIBIIICVI2 semiconductors with a chalcopyrite crystal structure would meet these criteria if 6Li is included, in accordance with the present invention.
The primary competition to such ternary AIBIIICVI2 semiconductors is from CLYC, a Cs2LiYCl6:Ce scintillator that detects both thermal neutrons (via the 6Li(n,α) 3He reaction) and gamma-rays, with an energy resolution of 3.38%. While the theoretical limit for energy resolution with scintillators is 2%, wide-band gap semiconductors, such as CdZnTe, have been demonstrated to be producible in large quantities, with 2% energy resolution at 662 keV. Furthermore, wide-band gap semiconductors may have improved photopeak efficiency as compared to scintillators, due to high-Z loading. For instance, LiInSe has a density of 4.49 g/cm3 (as compared to 3.67 g/cm3 for NaI or 3.31 g/cm3 for CLYC, a scintillating thermal neutron detector) and is constituted of elements with Z values of 3 (Li), 49 (In), and 34 (Se). Clearly, making a detector sensitive to slow neutrons by including Li means that it will be somewhat less efficient for sensing energetic gamma rays, but provides an opportunity to sense both types of radiation when properly designing the detectors, in accordance with the present invention.